The present invention relates to ballistic chronographs, which are devices for measuring the velocity of a projectile such as a bullet or pellet fired from a small arm or rifle. More particularly, the invention relates to systems which utilize the interruption of light for determining an elapsed time of the projectile between spaced intercepts, the velocity being inversely proportional to the elapsed time.
The general class of ballistic velocity determining apparatus for which this invention and those of the prior art relate use two or more light screens or beams which are separated by a measured or predetermined distance and are aligned orthogonally with respect to the path of the projectile. Velocity as used herein is defined as the time rate of change in distance that an object traverses, as it travels through space. The functional relationship is velocity equals distance/time. In the following description the velocity determined is the average velocity measured between two points and is not the instantaneous velocity determined at a single point in space. It is assumed that for the purpose of measuring muzzle velocities that the deceleration experienced by the projectile is constant over the distance for which the measurement is made. That is, from the instant that the projectile leaves the muzzle until it passes the far sensor. The term light is used in its broadest context, infrared, visible light or ultraviolet. The interruption of the light beams is sensed by photodetectors mounted in the forward and aft sensors which in turn produce signals which start and subsequently stop a counter which accumulates the number of clock pulses produced by an oscillator of known frequency. The velocity is then determined by dividing the distance by the elapsed time being the accumulated or counted number of clock pulses times the period or time interval per pulse.
A variety of light beams or screens are utilized in the chronographs of the prior art. U.S. Pat. Nos. 4,239,962 and 4,845,690 to Oehler disclose photodetector assemblies making use of diffuse ambient light. U.S. Pat. No. 4,272,189 to Bailey et al. discloses the use of pairs of linear arrays of light sources, photoemitters, and photodetectors mounted in a plane orthogonally to each other and to the path of the projectile. U.S. Pat. No. 4,574,238 to Weinlich discloses an apparatus which places facing photoemitters and photodetectors mounted behind optical slip apertures utilizing cylindrical lenses in a rectangular prism enclosure. U.S. Pat. No. 4,155,647 to Michel uses an expanded laser beam optically collimated in a thin beam which is reflected from two planar reflectors which in turn fold the beam back in the direction from which it was generated, thus producing two light screens. U.S. Pat. No. 5,577,733 to Downing discloses a light screen using a pair of light sources and associated linear arrays of detectors for detecting coordinates of passage of the projectile in a plane, in combination with an additional detector array in an axially spaced plane for determining projectile velocity. Other issues addressed in the above and other patents directed to ballistic velocity measurements relate to digital signal processing that is performed on the accumulated count, such as calculation and data presentation of average velocity, maximum and minimum velocities or extreme spread, and the standard deviation of a data set.
U.S. Pat. No. 4,128,761 Oehler discloses the use of a logarithmic amplifier and AC coupling for extending the dynamic range photodetectors using ambient lighting. Since the accuracy of a ballistic chronograph begins and ends with its sensor assemblies and detection electronics, the primary emphasis of the present invention is placed on these functions and not on the readout and data processing functions. U.S. Pat. No. 3,624,401 to Stoller discloses an optical scoring system using UV light reflected from passing projectiles.
U.S. Pat. No. 3,487,226 to Yetter et al. discloses a method and apparatus for determining the coordinate of a projectile by measuring the time interval between the interception of successive light screens with a predisposed spatial orientation. The light screens are formed through the use of planar arrays of subminiature incandescent lamps, with collimating lenses and a slit aperture. The photodetection array consists of a planar array of photodiodes, shown as phototransistors, with a corresponding slit aperture and collimating lens. The photodetector circuit consists of a plurality of photodetectors biased by an operating point bias stabilization circuit which is AC coupled to a two stage, AC coupled, pulse amplifier which in turn drives a Schmitt Trigger circuitry. Although the primary function of the apparatus is to determine the location of the bullet path in space, velocity can be determined from the intercept times between the parallel screens located at the entry and exit planes.
The velocity measurement schemes of the prior art typically exhibit one or more of the following disadvantages:
1. The circuitry is excessively complex for accommodating a large dynamic range in photocurrent when ambient lighting is used; PA1 2. They are unreliable due to excessive false triggering, particularly when ambient lighting is used; PA1 3. They are ineffective in that there is no clear delineation of the physical extent of the detection zone; PA1 4. They are difficult to use in that the detection space is undesirably small; and PA1 5. They are inaccurate, having poor transient response due to high parasitic capacitance of parallel photodiode arrays. PA1 1. Ballistic velocity measurement accuracies with RMS errors that are an order of magnitude better than that which is commercially available; PA1 2. Velocity measurement of bullets or projectiles ranging from 0.177 inch caliber pellets to the full range of small arm and high-powered rifle calibers and corresponding velocities; PA1 3. Operation independently of ambient lighting that can vary from indoor incandescent and fluorescent lighting to outdoor overcast and bright sunlight conditions; PA1 4. Highly repeatable measurements over a long period of time for monitoring cartridge fabrication quality and/or performance including the performance of individual components of a cartridge; PA1 5. Verifiable field calibration with a minimum of instrumentation; PA1 6. Very low false data acquisition rates; PA1 7. Immunity from muzzle blast, muzzle flash and shock wave effects of bullets traveling at supersonic velocities; PA1 8. A relatively large aperture for relaxed aiming requirements; and PA1 9. To provide for light screens having well defined and easily determined vertical, horizontal, and thickness (axial) boundaries. PA1 (a) providing an assembly including a spaced pair of sensor units, each sensor unit having a radiation emitter and sensor, respective optical paths between the emitters and detectors intersecting a ballistic path for being sequentially interrupted by a projectile, the radiation emitters being electrically activated in response to an external signal; PA1 (b) providing a timer for measuring a time interval between sequential interruptions of the optical paths in response to signals from the detectors, thereby to produce a measured function of an average velocity of the projectile; PA1 (c) simultaneously electrically interrupting the activation of the emitters for producing a test interval output of the emitters and a corresponding test output of the timer; and PA1 (d) offsetting a subsequently measured time interval being responsive to the projectile negatively by the test interval output for producing a corrected measured function of the average projectile velocity. PA1 (a) determining whether the test output of the timer is zero; and PA1 (b) if so: interchanging connections between the detectors and the timer; repeating the step of interrupting for producing a new test output; restoring the connections between the detectors and the timer; and using the new test output oppositely in the step of offsetting. PA1 (a) providing a rod having a thickness being less than the thickness of each optical path; PA1 (b) providing for each of the sensors a non-saturated test signal being responsive to the sensor; PA1 (c) holding the rod oriented perpendicular to the ballistic path; PA1 (d) moving the rod in the direction of the ballistic path between opposite sides of each optical path for partially blocking the optical paths; PA1 (e) during the step of moving the rod, monitoring corresponding test signal; PA1 (f) determining respective locations of the rod in the direction of the ballistic path corresponding to maximum blockage of the optical paths at corresponding singularities of the test signals; and PA1 (g) measuring a distance between the locations in the direction of the ballistic path.
Thus there is a need for a chronograph apparatus that overcomes the disadvantages of the prior art, that is simple, direct, inexpensive to produce and easy to use, while providing one or more of the following: